This invention relates to an optically isolated logical contention bus and more specifically to a communication system having the electrical ground of both a transmitter and a receiver electrically isolated from the electrical ground of a communicating bus therebetween.
As employed in a data communication system, a logical contention bus is generally capable of supplying two states, i.e. logical 1 and logical 0, which are typically characterized by two different bands of voltage levels. For example, in transistor-transistor logic, or TTL logic, a logical 1 is typically guaranteed by voltage within the band of 2.0 to 5.0 volts and a logical 0 is typically guaranteed by voltage within the band of 0.0 to 0.8 volts. Further, if a plurality of transmitters are coupled to the bus, one state is guaranteed to prevail in the presence of the other state where there is concurrently at least one transmitter placing a logical 1 and at least one transmitter placing a logical 0 on the bus. The guaranteed state is called the superior state and the other state is called the inferior state. Since it is relatively easy to shunt current in the bus to create the superior state, generally, for the superior state, current does not flow in the bus between transmitters and receivers coupled to the bus and for the inferior state, current flows in the bus between transmitters and receivers coupled to the bus.
In distributed communication systems, there are often large distances between processing components of the system and thus the medium comprising the communication bus, e.g. coaxial cable, twisted-pair wires, etc., must extend over these distances in order to interconnect the components. Typically, each station or group of physically close (with respect to the overall length of the bus) components comprises a transmitter and receiver, capable of placing data onto and removing data from the bus, respectively, coupled to the communication bus. Further, power supplies required to provide operating voltages to components at each station are generally connected to an electrical ground in the vicinity of each station. In hostile environments, such as near high power electrical machinery causing electromagnetic interference, large voltage transients, typically several thousand volts, (e.g. up to about 3 kV) may be coupled onto the bus, and thereby may damage expensive low-voltage signal or data generating circuitry connected to the bus. Further, a potential difference may exist between a local and a remote electrical ground thereby causing ground current to flow through the communication bus between the local and remote electrical grounds. Ground currents cause voltage shifts at the receiver which may invert the expected voltage across the receiver load. Further, large voltage shifts at the receiver input may be sensed by, and overstress, the input transformers of the power supplies causing dielectric breakdown thereof. Additionally, the potential difference between local and remote grounds may randomly vary independently over time, making compensation therefor difficult. Attempts to connect all station components in the distributed system directly to a common electrical ground would require extensive wiring or cabling along with the additional cost therefor and in some cases, such as factory automation, may not be feasible because a plurality of ground loop current paths would be created.
It would be desirable to provide a distributed data communication system wherein the data communication bus is electrically isolated from transmitters and receivers coupled thereto such that voltage transients on the bus and ground currents from the bus do not reach the circuitry of the transmitters and receivers.
Accordingly, it is an object of the present invention to provide a distributed communication system wherein a communication bus is responsive to and electrically isolated from transmitters and receivers coupled thereto.
Another object of the present invention is to provide electrical isolation between a communication bus having transients of many thousands of volts, e.g. greater than about 3 kV, and transmitters and receivers coupled thereto.
Still another object of the present invention is to reduce the effects of ground currents on the communication network.